In order to more fully appreciate the cluster tool of the present invention, some discussion of the scientific principles behind a vacuum deposition process is helpful. In general, if two electrodes and the gas therebetween are at extreme low pressure, less than 1/10,000 of atmospheric pressure, the resistance of the gas will break down and allow current flow between the two electrodes. This is called a "glow discharge" in the prior art.
Importantly, during the glow discharge process, the ionized gas atoms become attracted to and collide with the negative electrode (cathode), while the free electrons become attracted to and move towards the positive electrode (anode). Since electron mass is generally negligible relative to an atom, the electrons do not substantially affect the anode during collision. The mass of the ionized gas atoms, however, may be substantial relative to the mass of the atoms for the cathode material. Accordingly, when the ionized gas atoms collide with the cathode, the force of collision causes cathode material to be emitted. This phenomena is known as sputtering, or the removal of material from the cathode by ion bombardment during a glow discharge.
The material that sputters from the cathode coats the surrounding surfaces. If a sputtering device is designed in a certain manner, a uniform, quality coating of the sputtered material can be deposited onto a substrate. The deposition of sputter material to form such a coating is often referred to as a vacuum deposition process.
In the semiconductor industry, the vacuum process is used to coat silicon wafers, and certain devices that accomplish this result are known in the industry as cluster tools. To accomplish the vacuum process, however, the silicon wafer must be processed within a cluster tool capable of maintaining an extreme vacuum condition to ensure film purity. In addition to the vacuum issue, all water vapor molecules must be removed from the wafer to prevent contamination of the vacuum processes within the cluster tool, as water vapor is an undesirable process component that affects the quality of the finished wafer. Finally, outgoing wafers may be hot after undergoing a plurality of vacuum processes, and it is desirable to cool the wafers within the cluster tool, to reduce the risk of oxidation or corrosion.
Cluster tools typically consist of a loadlock chamber for receiving wafers and a plurality of vacuum processing chambers for processing the wafers. In a conventional cluster tool, the wafers are introduced to the loadlock in a 25-wafer cassette module. Typically, the entire cassette module of wafers is placed in the loadlock chamber, and the loadlock chamber (along with the cassette module) is pumped down to high vacuum. This is a time consuming process. If the volume in the loadlock vacuum chamber could be minimized, and the vacuum condition established is merely a "rough" vacuum sufficient for further processing of the wafer within the cluster tool, the time required to establish a vacuum on the loadlock chamber will be reduced. Thus, the efficiency of the cluster tool would be increased.
Prior art cluster tools typically have a dedicated degassing chamber for degassing the wafer, or removing the entrained water vapor molecules from the wafer. In the prior art, once the cassette module is under "rough" to mid-range vacuum conditions (10.sup.-2 torr to 10.sup.-3 torr, where 760 torr=1 Atm=atmospheric pressure), the wafers are successively placed in the dedicated degassing chamber for individual degassing prior to undergoing several vacuum processes. The dedicated degassing chamber is usually within the high vacuum body of the prior art cluster tool. What is desired is a cluster tool that degasses the wafers in the loadlock chamber under rough vacuum conditions. This eliminates the need for a dedicated processing position within the high vacuum body of the cluster tool.
Degassed water vapor is removed from the high vacuum body of the cluster tool by cryopumps. A cryopump typically consists of a refrigeration unit that maintains an extremely low temperature on an array of metallic plates. The array is positioned in communication with the chamber and the water vapor molecules therein, and the water molecules impinge on the plate and are frozen thereto. These cryopumps are expensive and difficult to maintain. What is desired is a cluster tool that eliminates the need for a dedicated cryopump for each degassing chamber. One way to eliminate a dedicated cryopump is to remove water vapor molecules from the wafer while the wafer is at rough vacuum in the loadlock chamber. This will reduce the amount of residual water vapor in the proximity of the high vacuum body of the cluster. If this can be accomplished, vacuum quality and vacuum process quality can be enhanced. Also, the required number of cryopumps for the cluster tool would decrease, which further decreases the maintenance requirements for the cluster tool and increases the overall reliability of the tool.
In the prior art, once each wafer from the cassette module has been degassed, the wafer is successively placed in a plurality of vacuum processing chambers and undergoes a plurality of vacuum processes. After completion of the processes, the wafer may be placed in a dedicated chamber for cooling. The wafer is then placed back into the cassette module, and another wafer is selected from the cassette to begin the processes discussed above. When all of the wafers in the cassette module have finished processing, the entire cassette module is removed from the loadlock chamber.
With the above configuration, however, the first wafer spends more time cooling after its process is completed than the final wafer. Conversely, the last wafer to be processed has spent more time "drying" under vacuum before undergoing its vacuum processes. This can lead to a drift in measurable process quality in the wafers being loaded one at a time from the cassette module into the cluster tool for processing. This is known in the prior art as the "first wafer effect". What is desired is a cluster tool with loadlock chambers that are capable of performing both degas and cooling functions during processing of the wafer, to standardize the amount of time each wafer spends at each step of the process and, thereby, to improve the consistency of the finished wafer quality.
U.S. Pat. No. 5,516,732, which issued to Flegal for an invention entitled "Wafer Processing Machine Vacuum Front End Method and Apparatus", discloses a station in which a preheating and degassing function are combined. Flegal, however, does not disclose a water pump positioned in fluid communication with the chamber for removal of the water vapor molecules at extreme vacuum. This is because the device, as disclosed by Flegal, is known as a "front end device" and must be connected to a cluster tool to perform a vacuum process. Accordingly, Flegal does not minimize the "water load" on the processing stations and does not minimize the number of cryopumps required to operate the cluster tool.
U.S. Pat. No. 5,902,088, which issued to Fairbairn et al for an invention entitled "Single Loadlock Chamber With Wafer Cooling Function", discloses a device in which a plurality of trays for receiving wafers are mounted within a loadlock structure, with the loadlock structure also performing a cooling function. The loadlock structure disclosed by Fairbairn, however, is sized to hold twelve to fourteen wafers, and establishing a vacuum on this size of a loadlock chamber presents the same problems that the present invention overcomes. Further, like Flegal, Fairbairn does not disclose a water pump attached in fluid communication with the loadlock structure and also does not minimize the number of water pumps required for the overall cluster tool.
In view of the above, it is an object of the present invention to provide a cluster tool that increases throughput of silicon wafers by performing the degassing of the wafer simultaneously with establishing a vacuum for the loadlock chamber containing the wafer. This decreases the overall processing time and increases wafer throughput.
It is another object of the present invention to provide a cluster tool that performs the degassing and the cooling steps within the loadlock chamber, to eliminate the need for separate dedicated degassing and cooling chambers. It is another object of the present invention to provide a cluster tool with a loadlock chamber that is sized to process a single wafer at a time, thereby minimizing the required volume to be placed under vacuum and eliminating what is known as "the first wafer effect".
It is another object of the present invention to provide a cluster tool with a single high vacuum chamber which services two loadlocks. This configuration minimizes the number of cryopumps required for the cluster tool and maximizes the reliability of the cluster tool.
Each of the above features and objectives results in a cluster tool that is easier to operate and is more reliable. It is also simpler to manufacture at less cost.